The present invention is a process for reducing organohalosilanes and volatile organohalopolysilanes. The process takes advantage of the high susceptibility of silicon-halogen bonds to alkylaluminum hydride reducing agents. The high susceptibility of the silicon-halogen bond to chemical reduction allows a process to be run where the resultant reduced product can be removed from a reaction mixture before additional or less favorable reactions can occur.
The process comprises forming a mixture of the halosilane, to be reduced, and a liquid alkylaluminum hydride reducing agent. The organohalosilane is only contacted with the alkylaluminum hydride long enough for the reactive silicon-halogen bond to be reduced. Temperature and pressure conditions of the process are controlled such that the boiling reduced product is caused to immediately vaporize from the mixture without undergoing further reaction.
The described process can be run to control the number of halogen atoms remaining on silicon. That is, both fully and partially reduced silanes can be produced. The process can be run as a one-step process requiring no additional separation steps to recover the reduced silanes.
The silicon-halogen bond is known to be a highly reactive bond that can be rapidly reduced to the corresponding silicon-hydrogen species in the presence of reducing agents. Generally, the more halogen atoms present on a particular silicon atom, the greater the reactivity of the silicon-halogen bond. For example, the order of reactivity for the silicon-chlorine bond is RSiCl.sub.3 &gt;R.sub.2 SiCl.sub.2 &gt;R.sub.3 SiCl where R is a hydrocarbon group.
Jenkner, U.S. Pat. No. 3,100,788, issued Aug. 13, 1963, teaches the reduction of organohalosilanes by reaction with hydrogen in the presence of alkali metals, or with alkali hydrides. The reaction is carried out at elevated temperatures in the range of 50.degree. C. to 400.degree. C. and pressures of 1 atm to 350 atm or more. One of the benefits Jenkner claims for this process is that very few compounds of Si-Si structure are produced.
Berger, U.S. Pat. No. 3,496,206, issued Feb. 17, 1970, describes a process whereby an alkali metal hydride is used in combination with an alkylaluminum halide, to effect the reduction of organohalosilanes. The preferred process employed a substantially inert organic solvent and a temperature between 100.degree. C. to 130.degree. C. The reduced product was subsequently distilled.
Robinson, U.S. Pat. No. 3,499,020, issued Mar. 3, 1970, teaches the reduction of halogen containing silanes with dialkylaluminum hydrides in the presence of an inert solvent and an ether. The process was ran under closed conditions and the reduced product subsequently isolated. Robinson teaches that care must be taken in the reduction of compounds containing unsaturated linkages to maintain the temperature substantially below 100.degree. C. in order to prevent the occurrence of addition reactions.
Schulz, Co-Pending U.S. application Ser. No. 07/531,616, filed 6/1/90, now U.S. Pat. No. 4,973,723, describes a process in which halosilacycloalkanes are reduced with an alkylaluminum hydride to produce silacycloalkanes. In the described process, the silacycloalkane is immediately vaporized from the mixture to prevent further reaction with the alkylaluminum hydride.
None of the cited art recognizes that the uniquely reactive nature of the silicon-halogen bond present in organohalosilanes and organohalopolysilanes makes possible a process in which the bond can be reduced and the reduced product removed from the mixture before other, less favorable, processes can occur.